Re: [PATCH] btrfs: do not account twice for inode ref when reserving metadata units

[Nikolay Borisov <nborisov@suse.com>]

On 9.05.22 г. 18:29 ч., fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> When reserving metadata units for creating an inode, we don't need to
> reserve one extra unit for the inode ref item because when creating the
> inode, at btrfs_create_new_inode(), we always insert the inode item and
> the inode ref item in a single batch (a single btree insert operation,
> and both ending up in the same leaf).
> 
> As we have accounted already one unit for the inode item, the extra unit
> for the inode ref item is superfluous, it only makes us reserve more
> metadata than necessary and often adding more reclaim pressure if we are
> low on available metadata space.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

That's a neat little optimisation.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>   fs/btrfs/inode.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index b42d6e7e4049..adc8b684fe31 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -6138,12 +6138,15 @@ int btrfs_new_inode_prepare(struct btrfs_new_inode_args *args,
>   		(*trans_num_items)++;
>   	} else {
>   		/*
> -		 * 1 to add inode ref
>   		 * 1 to add dir item
>   		 * 1 to add dir index
>   		 * 1 to update parent inode item
> +		 *
> +		 * No need for 1 unit for the inode ref item because it is
> +		 * inserted in a batch together with the inode item at
> +		 * btrfs_create_new_inode().
>   		 */
> -		*trans_num_items += 4;
> +		*trans_num_items += 3;
>   	}
>   	return 0;
>   }